Ashrae Journal - December 2008 - (Page 32) is depicted in the schematic. The energy recovery efficiency in this illustration is based on the use of recovery wheels of a molecular sieve design. Hot, humid air is cooled and dehumidified by the first air-to-air total energy wheel. A cooling coil provides mechanical refrigeration to add the remaining required dehumidification. This coil is controlled during summer operation based on maintaining the average relative humidity in the building, as the coil discharge temperature is reset up from the minimum temperature conditions of 50°F (10°C) dry bulb and 49.5°F (9.7°C) wet bulb. The second air-to-air energy recovery device, a sensible wheel, provides efficient reheat to deliver the dehumidified air at 68°F (20°C) to the chilled beams. The beauty of this design is that the second heat recovery device, while providing efficient reheating, also greatly improves the cooling efficiency of the first energy recovery device. As indicated in the schematic, the preheating and final heating coils in the airhandling unit are off during peak summer operation. Figure 6 illustrates the system operation during peak winter operation. The preheating and final heating coils provide additional heat to the airstream as required, protecting the first wheel from frosting, and compensating for less than 100% energy recovery efficiency. The cooling coil for mechanical refrigeration during summer cooling is normally off during winter operation. In this example, the second heat recovery wheel is assumed to be off as well. However, it would be possible for both wheels to operate to share the preheat load to attain maximum recovery. In an effort to minimize the energy used for reheating and the recooling effect at the beams, the building management system can include a sequence of operation during winter operation to reset the discharge temperature from the air-handling unit below 68°F (20°C) based on the laboratory space requiring the least cooling (which is the space most vulnerable to overcooling due to cold primary air entering the beams). In the case of the NJEDA Tech IV Building, to eliminate concerns over cross-contamination, and because there are not many hoods, the hood exhaust is taken out through a separate exhaust system and does not pass through the wheels. This results in a slight imbalance of airflow and, loss of some energy recovery effectiveness. However, this effect is minimal on total recovery (sensible and latent) with an exhaust to supply ratio of at least 70%. As an alternate approach it might be possible to exhaust some or all of the hoods through the energy wheels, but this must be evaluated on a hood-by-hood basis, in relation to the type of wheel used, due to potential safety issues. (Refer to Reference 5, which addresses the minimum expected crosscontamination expected with a wheel of 3-A molecular sieve design. It is not the intent of this article to focus in-depth on this potential safety issue.) It would also be possible to use heat pipes to protect against cross-contamination in the air-to-air energy recovery devices and eliminate the separate hood exhaust system but at less recovery efficiency. In an effort to maximize energy savings, wheels of a 3-A molecular sieve design serve as the method of heat recovery in this analysis. As indicated previously, Figures 5 and 6 indicate that a supplemental supply of neutral air is provided to each space 32 ASHRAE Journal 100 ft2 Block of Laboratory Space (Plan View) 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.5 >0.4 m/s 1.0 1.5 0.3 – 0.4 m/s Elevation 2.0 2.5 3.0 0.0 0.2 – 0.3 m/s (39 – 58 fpm) 2.5 2.0 1.5 1.0 0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 Figure 4: Typical air distribution. through separate air devices. Space pressurization is maintained through the use of air terminal units in the general exhaust, supplemental air supply, and air supply to the chilled beams. In the absence of skin losses for the interior laboratory spaces in the NJEDA Tech IV Building under winter operation, the chilled beams are two-pipe units for cooling only. It is anticipated that some lighting and equipment will be operating at all times, precluding overcooling at any time with primary air at 68°F (20°C). Alternately, hot water coils can be included in the same air terminal units used to maintain space pressurization. ashrae.org December 2008 http://www.ashrae.org
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